The present invention relates to a lighting device (also called a backlight) to be installed on back surface of a liquid crystal display for illuminating said liquid crystal display from backside. In particular, the invention relates to a surface light source device, which does not cause uneven brightness.
In a transmission type liquid crystal display, a backlight for projecting an illuminating light of planar shape is installed on back surface of a liquid crystal panel for forming an electronic latent image. Here, the backlight of this type is called “a surface light source device”. In this surface light source device, a plurality of line light sources are arranged in parallel to the back surface of the liquid crystal panel on a reflecting plate for effectively utilizing exit lights from the line light sources. It is generally practiced to have smooth and even in-plane distribution of the illuminating light by providing a light diffusion plate positioned between the liquid crystal panel and light source device. A polarizing plate is laminated on upper surface and lower surface of the liquid crystal panel, and the surface light source device is installed on back surface of the polarizing plate on lower side.
As a reflecting plate to be used in the surface light source device as described above, the Patented Reference 1 as given below describes a reflecting mirror for backlight, which comprises two half-ellipses aligned with each other and commonly sharing a first focal point, and second focal points are arranged one on each side of a straight line including the first focal point. By placing a light source at the first focal point, an image of the light source is formed at the second focal point so that it is seen as if there are three light sources.
[Patented Reference 1] JP-A-2005-24746
The Patented Reference 1 describes a reflecting mirror, which comprises two half-ellipses aligned with each other and reflects a light from a light source by specular reflection (or mirror reflection). However, this reference gives no description on a problem of uneven brightness at the boundary of the two half-ellipses.
It is an object of the present invention to provide a surface light source device in a lighting system, which comprises two half-ellipses aligned with each other and has a reflecting mirror for reflecting a light from the light source by specular reflection (or mirror reflection), and by which uneven brightness at a boundary of the two ellipses is eliminated.
To attain the above object, according to the present invention, it is provided a reflecting plate designed in concave form by aligning two half-ellipses to enclose the light source, and the reflecting plate is made of a diffused reflection material for reflecting the light including the specular reflection light and to eliminate uneven brightness by converging the specular reflection light to the boundary.
To converge the specular reflection light, the first focal point and the second focal points are positioned almost at the same height, and the position of the light source is shifted from the first focal point toward the reflecting plate. Also, the light source and the first focal point are arranged at positions closer to the reflecting plate. The second focal points are positioned near the boundary, and these positions are adjusted.
According to the present invention, a surface light source device can be provided, by which uneven brightness at the boundary of the two ellipses can be eliminated. The device according to the present invention is suitable as a surface light source device for a planar lighting system such as a ceiling lamp—as a backlight for liquid crystal display, as a backlight for transmission type signboard, or as a backlight for medical application to examine radiogram.
In a surface light source device using a reflecting mirror for backlight described in the background art, transmissivity is not more than 60% for the purpose of providing even directivity as the diffusion plate. According to the present invention, a diffusion plate with transmissivity of not more than 65% can be used. If a diffusion plate with high transmissivity can be used, it is possible to have brighter surface light source. This may be attributed to the fact that high directivity can be assured under the diffusion plate because not only the specular reflection light but also diffused reflection light can be used.